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3-Coloring and Other Elementary Invariants of Knots

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3-Coloring and Other Elementary Invariants of Knots KNOT THEORY BANACH CENTER PUBLICATIONS, VOLUME 42 INSTITUTE OF MATHEMATICS POLISH ACADEMY OF SCIENCES WARSZAWA 1998 3-COLORING AND OTHER ELEMENTARY INVARIANTS OF KNOTS JOZEF´ H. PRZYTYCKI Department of Mathematics, George Washington University, Washington, D.C. 20052, U.S.A. E-mail: [email protected] Classical knot theory studies the position of a circle (knot) or of several circles (link) in R3 or S3 = R3 . The fundamental problem of classical knot theory is the classification ∪∞ of links (including knots) up to the natural movement in space which is called an ambient isotopy. To distinguish knots or links we look for invariants of links, that is, properties of links which are unchanged under ambient isotopy. When we look for invariants of links we have to take into account the following three criteria: 1. Is our invariant easy to compute? 2. Is it easy to distinguish elements in the value set of the invariant? 3. Is our invariant good at distinguishing links? The number of components of a link, com(L), is the simplest invariant. A more in- teresting link invariant is given by the linking number, defined in 1833 by C. F. Gauss using a certain double integral [Ga]. H. Brunn noted in 1892 that the linking number has a simple combinatorial definition [Br]. Definition 0.1. Let D be an oriented link diagram. Each crossing has an associated sign: +1 for and 1 for . The global linking number of D, lk(D), is defined to be − half of the sum of the signs of crossings between different components of the link diagram. If the diagram has no crossings, we put lk(D) = 0. To show that a function defined on diagrams of links is an invariant of (global isotopy) of links, we have to interpret global isotopy in terms of diagrams. This was done by Reidemeister [Re, 1927] and Alexander and Briggs [A-B, 1927]. 1991 Mathematics Subject Classification: 57M. Supported by USAF grant 1-443964-22502 while visiting the Mathematics Department, U.C. Berkeley. The paper is in final form and no version of it will be published elsewhere. [275] 276 J. H. PRZYTYCKI Theorem 0.2 (Reidemeister theorem). Two link diagrams are ambient isotopic if ±1 and only if they are connected by a finite sequence of Reidemeister moves Ri ,i =1, 2, 3 (see Fig. 0.1 ) and isotopy (deformation) of the plane of the diagram. The theorem holds also for oriented links and diagrams. One then has to take into account all possible co- herent orientations of diagrams involved in the moves. or R 1 R 2 R 3 R 3 Fig. 0.1 Exercise 0.3. Show that lk(D) is preserved by Reidemeister moves on oriented link diagrams. Thus lk is an invariant of oriented links. Example 0.4. lk( ) = 0, lk( )= 1, lk( )= 1. Therefore the global − linking number allows us to distinguish the trivial link of two components, T2, the right- handed Hopf link, 21, and the left-handed Hopf link, 2¯1. 1. The tricoloring. The tricoloring invariant (or 3-coloring) is the simplest invariant which distinguishes between the trefoil knot and the trivial knot. The idea of tricoloring was introduced by R.Fox around 1960, [C-F, Chapter VI, Exercises 6-7], [F-2], and has been extensively used and popularized by J. Montesinos [Mon] and L. Kauffman [K]. 3-COLORING 277 Definition 1.1 ([P-1]). We say that a link diagram D is tricolored if every arc is colored r (red), b (blue) or y (yellow) (we consider arcs of the diagram literally, so that in the undercrossing one arc ends and the second starts; compare Fig. 1.1), and at any given crossing either all three colors appear or only one color appears. The number of different tricolorings is denoted by tri(D). If a tricoloring uses only one color we say that it is a trivial tricoloring. Fig. 1.1. Different colors are marked by lines of different thickness. Lemma 1.2. The tricoloring is an (ambient isotopy) link invariant. P r o o f. We have to check that tri(D) is preserved under the Reidemeister moves. The invariance under R1 and R2 is illustrated in Fig. 1.2 and the invariance under R3 is illustrated in Fig. 1.3. Fig. 1.2 Fig. 1.3 Because the trivial knot has only trivial tricolorings, tri(T1) = 3, and the trefoil knot allows a nontrivial tricoloring (Fig. 1.1), it follows that the trefoil knot is a nontrivial knot. Exercise 1.3. Find the number of tricolorings for the trefoil knot (31), the figure eight knot (41) and the square knot (31#3¯1, see Fig. 1.4). Then deduce that these knots are pairwise different. 278 J. H. PRZYTYCKI Lemma 1.4. tri(L) is always a power of 3. Proof. Denote the colors by 0, 1 and 2 and treat them modulo 3, that is, as elements of the group (field) Z3. All colorings of the arcs of a diagram using colors 0, 1, 2 (not r necessarily allowed 3-colorings) can be identified with the group Z3 where r is the number of arcs of the diagram. The (allowed) 3-colorings can be characterized by the property that at each crossing the sum of the colors is equal to zero modulo 3. Thus (allowed) r 3-colorings form a subgroup of Z3 . The elementary properties of tricolorings, which we give in Lemma 1.5, follow imme- diately from the connections between tricolorings and the Jones and Kauffman polyno- mials of links. We will give here an elementary proof of (a)-(c) of Lemma 1.5. There is also an elementary proof of (d) (based on the flow-potential idea of Jaeger, see [Ja-P]), but it is more involved; compare Lemma 2.2. Lemma 1.5. (a) tri(L1)tri(L2)=3tri(L1#L2), where # denotes the connected sum of links (see Fig. 1.4 ). (b) Let L+,L−,L0 and L∞ denote four unoriented link diagrams as in Fig. 1.5. Then, among four numbers tri(L+),tri(L−),tri(L0) and tri(L∞), three are equal one to another and the fourth is equal to them or is 3 times bigger. In particular: (c) tri(L+)/tri(L−)=1 or 3, or 1/3. Part (b) can be strengthened to show that: (d) Not all the numbers tri(L+),tri(L−),tri(L0) and tri(L∞) are equal. LL 1 2 - L1 #L 2 31 #31 Fig. 1.4 Fig. 1.5 P r o o f. (a) An n-tangle is a part of a link diagram placed in a 2-disk, with 2n points on the disk boundary (n inputs and n outputs); Fig. 1.6. We show first that for any 3-coloring of a 1-tangle (i.e. a tangle with one input and one output; see Fig. 1.6(a)), the input arc have the same color as the output arc. Namely, let T be our 3-colored tangle and let the 1-tangle T ′ be obtained from T by adding a trivial component, C, below T , 3-COLORING 279 close to the boundary of the tangle, so that it cuts T only near the input and the output; Fig. 1.6(b). Of course the 3-coloring of T can be extended to a 3-coloring of T ′ (in three different ways), because the tangle T ′ is ambient isotopic to a tangle obtained from T by adding a small trivial component disjoint from T . If we, however, try to color C, we see immediately that it is possible iff the input and the output arcs of T have the same color. x4x 3 x1 x2 (a) (b) (c) T T' TD Fig. 1.6 Thus if we consider a connected sum L1#L2, we see from the above that the arcs joining L1 and L2 have the same color. Therefore the formula, tri(L1)tri(L2) = 3tri(L1#L2), follows. (b) Consider a crossing p of the diagram D. If we cut out of D a neighborhood of p, we are left with the 2-tangle, TD (see Fig. 1.6(c)). The set of 3-colorings of TD, Tri(TD), forms a Z3 linear space. Each of the sets of 3-colorings of D+,D−,D0 and D∞, Tri(D+),Tri(D−),Tri(D0) and Tri(D∞), respectively form a subspace of Tri(TD). Let x1, x2, x3, x4 be generators of Tri(TD) corresponding to arcs cutting the boundary of the tangle; see Fig. 1.6(c). Then any element of Tri(T ) satisfies the equality x x + D 1 − 2 x x = 0. To show this, we proceed as in part (a). Any element of Tri(D ) (resp. 3 − 4 + Tri(D−), Tri(D0) and Tri(D∞)) satisfies additionally the equation x2 = x4 (resp. x1 = x3, x1 = x2 and x1 = x4). Thus Tri(D+) (resp. Tri(D−), Tri(D0) and Tri(D∞)) is a subspace of Tri(TD) of codimension at most one. Let F be the subspace of Tri(TD) given by the equations x1 = x2 = x3 = x4, that is the space of 3-colorings monochromatic on the boundary of the tangle. F is a subspace of codimension at most one in any of the spaces Tri(D+), Tri(D−), Tri(D0), Tri(D∞). Furthermore the common part of any two of Tri(D+), Tri(D−),Tri(D0),Tri(D∞) is equal to F . To see this we just compare the defining relations for these spaces. Finally notice that Tri(D ) Tri(D−) Tri(D ) + ∪ ∪ 0 ∪ Tri(D∞)= Tri(TD).
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